Cigarette usage is being assaulted from every direction. It is being heavily regulated, Taxed and maligned with it's own money. The status of cigarette usage and the various and diverse effort to eradicate it ought to be a case study for how any sort of prohibition might be enacted with minimal backlash.
Had the Alcohol prohibitionists followed a similar route they may have eventually achieved their objective. They made the mistake of pushing too hard and too fast. The Backlash did them in.
But the effort to eradicate cigarettes will not likely work with stronger drugs. While the cravings of cigarette smokers are persistent, they are nothing as compared to the cravings of a crack head or heroin user.
As for the drop in Alcohol usage, (assuming your numbers are correct) I can only surmise that those users substituted other things.
Surmising, imagining, and assuming are staples of the pro-drug-war position.